Inflatable airbags have been well accepted for use in motor vehicles and have been credited with preventing numerous deaths and injuries. Some statistics estimate that frontal airbags reduce the fatalities in head-on collisions by 25% among drivers using seat belts and by more than 30% among unbelted drivers. Statistics further suggest that with a combination of seat belt and airbag, serious chest injuries in frontal collisions can be reduced by 65% and serious head injuries by up to 75%.
A modern airbag apparatus may include an electronic control unit (ECU) and one or more airbag modules. The ECU is usually installed in the middle of an automobile, between the passenger and engine compartments. If the vehicle has a driver airbag only, the ECU may be mounted in the steering wheel. The ECU includes a sensor which continuously monitors the acceleration and deceleration of the vehicle and sends this information to a processor which processes an algorithm to determine if the vehicle is in an accident situation.
When the processor determines that there is an accident situation, the ECU transmits an electrical current to an initiator in the airbag module. The initiator triggers operation of the inflator or gas generator which, in some embodiments, uses a combination of compressed gas and solid fuel. The inflator inflates a textile airbag that cushions a passenger during impact to prevent injury to the passenger. In some airbag systems, the airbag may be fully inflated within 50 thousandths of a second and deflated within two tenths of a second.
Airbag systems have been primarily designed for deployment in front of an occupant, between the upper torso and head of an occupant and the windshield or instrument panel. Conventional airbags, such as driver or passenger airbags, protect the occupant's upper torso and head from colliding with a windshield or instrument panel.
Airbag technology has advanced to include airbag systems that protect occupants during a side impact or roll-over accident. In these accidents, the occupant may be thrown against the windows, doors and side-walls of the vehicle. These airbag systems are known as curtain airbags. Generally, the curtain airbag is attached to a thin long frame member that runs along a side of the roof of the vehicle.
Typically, the airbag of a curtain airbag system inflates and descends from the frame member to cover a majority of the area between the occupant and the side of the vehicle interior. The inflated airbag appears much like a curtain covering the vehicle window. The curtain airbag may protect the occupant from impact with a side window, flying shards of glass, and other projectiles. The curtain airbag may also serve to keep the occupant inside the vehicle during a roll-over accident.
Generally, the curtain airbag is installed in a very limited thin space, defined by the roof frame member. The inflator may be a thin, cylindrical member that extends a portion of the length of the curtain airbag. In this manner, the curtain airbag inflator is capable of providing sufficient inflation gas to properly inflate the curtain airbag. The gas is created from the rapid burning of pyrotechnic materials. The gas escapes exit ports in the inflator at a high velocity and temperature. Due to the limited space, the textile bag is generally stored by folding against the inflator.
An airbag cover, also called a trim cover panel, covers a compartment containing the airbag module and may reside on a steering wheel, dashboard, door, along a vehicle roof rail, vehicle wall, vehicle pillar or beneath the dash board. The airbag cover is typically made of a rigid plastic and may be forced open by the pressure from the deploying airbag. In deploying the airbag, it is preferable to retain the airbag cover to prevent the airbag cover from flying loose in the passenger compartment. If the airbag cover freely moves into the passenger compartment, it may injure a passenger. Also, to insure there will be no flying fragments ejected into the passenger compartment a cloth “scrim” is required on the back of the part to keep in fragments in place.
Interior trim cover panels currently used in connection with airbag systems are generally made of very soft and flexible rubber modified materials, in order to withstand the impact and bending of such parts during airbag deployment. As discussed above, when the airbag deploys, the interior trim cover panels must be able to withstand the impact and flex out of the way for proper bag deployment. Currently, rubber modified polypropylenes having a flexural modulus of 1000 MPA or less are the only polypropylene-based materials that are utilized. However, these products create other issues. The low flexural modulus of the material has to be countered by reinforcing the part with ribs or other types of reinforcements. With lower modulus materials the heat distortion temperature is also compromised, which causes fit and finish issues when parts are tested at or exposed to elevated temperatures.
Polyolefins have seen limited use in engineering applications due to the tradeoff between toughness and stiffness. For example, polyethylene is widely regarded as being relatively tough, but low in stiffness. Polypropylene generally displays the opposite trend, i.e., is relatively stiff, but low in toughness.
Several well known polypropylene compositions have been introduced which address the toughness issue. For example, it is known to increase the toughness of polypropylene by adding rubber particles, either in-reactor resulting in impact copolymers, or through post-reactor blending. However, while toughness is improved, stiffness is considerably reduced using this approach.
In the molding of automobile parts, such as interior trim cover panels, injection molding and compression molding processes have been employed. Injection molding of thermoplastic resin has been used for many small articles. Thermosetting polyester filled with chopped fibers has been compression molded into relatively large sheets or panels. Despite many attempts to produce interior trim cover panels having a high quality surface finish, the surface finish obtained is not particularly good.
Glass reinforced polypropylene compositions have been introduced to improve stiffness. However, the glass fibers have a tendency to break in typical injection molding equipment, resulting in reduced toughness and stiffness. In addition, glass reinforced products have a tendency to warp after injection molding.
Thermoplastic resins employing glass fibers have been extruded in sheet form. Glass fibers have also been used as a laminate in thermoplastic resin sheet form. The sheets can then be compression molded to a particular shape. As may be appreciated by those skilled in the art, compression molding has certain limitations since compression molded parts cannot be deeply drawn and thus must possess a relatively shallow configuration. Additionally, such parts are not particularly strong and require structural reinforcements when used in the production of vehicle body panels. Moreover, the surface finish of glass-filled resins is generally poor.
The automotive industry generally requires that all surfaces visible to the consumer have “class A” surface quality. Components made of glass-filled compositions often require extensive surface preparation and the application of a curable coating to provide a surface of acceptable quality and appearance. The steps required to prepare such a surface may be expensive and time consuming and may affect mechanical properties.
Although the as-molded surface quality of glass-filled components continues to improve, imperfections in their surfaces due to exposed glass fibers, glass fiber read-through, and the like often occur. These surface imperfections may further result in imperfections in coatings applied to such surfaces. Defects in the surface of glass-filled compositions and in-cured coatings applied to the surfaces of glass-filled compositions may manifest as paint popping, high long- and short-term wave scan values, orange peel, variations in gloss or the like.
Several techniques have been proposed to provide surfaces of acceptable appearance and quality. For example, overmolding of thin, preformed paint films may provide a desired Class A surface. However, such overmolding is usually applicable only for those compositions capable of providing virgin molded surfaces that do not require any secondary surface preparation operations. In-mold coating can obviate these operations, but only at the cost of greatly increased cycle time and cost. Such processes use expensive paint systems that may be applied to the part surface while the mold is re-opened slightly, and then closed to distribute and cure the coating.
As an alternative to the use of glass fibers, another known method of improving the properties of polyolefins is organic fiber reinforcement. For example, EP Patent Application No. 0397881, discloses a composition produced by melt-mixing 100 parts by weight of a polypropylene resin and 10 to 100 parts by weight of polyester fibers having a fiber diameter of 1 to 10 deniers, a fiber length of 0.5 to 50 mm and a fiber strength of 5 to 13 g/d, and then molding the resulting mixture. Also, U.S. Pat. No. 3,639,424 to Gray, Jr. et al., discloses a composition including a polymer, such as polypropylene, and uniformly dispersed therein at least about 10% by weight of the composition staple length fiber, the fiber being of man-made polymers, such as poly(ethylene terephthalate) (PET) Pr poly(1,4-cyclohexylenedimethylene terephthalate).
Fiber reinforced polypropylene compositions are also disclosed in PCT Publication WO 02/053629. More specifically, WO 02/053629 discloses a polymeric compound, comprising a thermoplastic matrix having a high flow during melt processing and polymeric fibers having lengths of from 0.1 mm to 50 mm. The polymeric compound comprises between 0.5 wt % and 10 wt % of a lubricant.
Various modifications to organic fiber reinforced polypropylene compositions are also known. For example, polyolefins modified with maleic anhydride or acrylic acid have been used as the matrix component to improve the interface strength between the synthetic organic fiber and the polyolefin, which was thought to enhance the mechanical properties of the molded product made therefrom.
Other background references include PCT Publication WO 90/05164; EP Patent Application 0669372; U.S. Pat. No. 6,395,342 to Kadowaki et al.; EP Patent Application 1075918; U.S. Pat. No. 5,145,891 to Yasukawa et al., U.S. Pat. No. 5,145,892 to Yasukawa et al.; and EP Patent 0232522, the entire disclosures of which are hereby incorporated herein by reference.
U.S. Pat. No. 3,304,282 to Cadus et al. discloses a process for the production of glass fiber reinforced high molecular weight thermoplastics in which the plastic resin is supplied to an extruder or continuous kneader, endless glass fibers are introduced into the melt and broken up therein, and the mixture is homogenized and discharged through a die. The glass fibers are supplied in the form of endless rovings to an injection or degassing port downstream of the feed hopper of the extruder.
U.S. Pat. No. 5,401,154 to Sargent discloses an apparatus for making a fiber reinforced thermoplastic material and forming parts therefrom. The apparatus includes an extruder having a first material inlet, a second material inlet positioned downstream of the first material inlet, and an outlet. A thermoplastic resin material is supplied at the first material inlet and a first fiber reinforcing material is supplied at the second material inlet of the compounding extruder, which discharges a molten random fiber reinforced thermoplastic material at the extruder outlet. The fiber reinforcing material may include a bundle of continuous fibers formed from a plurality of monofilament fibers. Fiber types disclosed include glass, carbon, graphite and Kevlar.
U.S. Pat. No. 5,595,696 to Schlarb et al. discloses a fiber composite plastic and a process for the preparation thereof and more particularly to a composite material comprising continuous fibers and a plastic matrix. The fiber types include glass, carbon and natural fibers, and can be fed to the extruder in the form of chopped or continuous fibers. The continuous fiber is fed to the extruder downstream of the resin feed hopper.
U.S. Pat. No. 6,395,342 to Kadowaki et al. discloses an impregnation process for preparing pellets of a synthetic organic fiber reinforced polyolefin. The process comprises the steps of heating a polyolefin at the temperature which is higher than the melting point thereof by 40 degree C. or more to lower than the melting point of a synthetic organic fiber to form a molten polyolefin; passing a reinforcing fiber comprising the synthetic organic fiber continuously through the molten polyolefin within six seconds to form a polyolefin impregnated fiber; and cutting the polyolefin impregnated fiber into the pellets. Organic fiber types include polyethylene terephthalate, polybutylene terephthalate, polyamide 6, and polyamide 66.
U.S. Pat. No. 6,419,864 to Scheuring et al. discloses a method of preparing filled, modified and fiber reinforced thermoplastics by mixing polymers, additives, fillers and fibers in a twin screw extruder. Continuous fiber rovings are fed to the twin screw extruder at a fiber feed zone located downstream of the feed hopper for the polymer resin. Fiber types disclosed include glass and carbon.
Application Ser. No. 11/318,363, filed Dec. 23, 2005, notes that consistently feeding PET fibers into a compounding extruder is a problem encountered during the production of polypropylene (PP)-PET fiber composites. Conventional gravimetric or vibrational feeders used in the metering and conveying of polymers, fillers and additives into the extrusion compounding process, while effective in conveying pellets or powder, are not effective in conveying cut fiber. Another issue encountered during the production of PP-PET fiber composites is adequately dispersing the PET fibers into the PP matrix while still maintaining the advantageous mechanical properties imparted by the incorporation of the PET fibers. More particularly, extrusion compounding screw configuration may impact the dispersion of PET fibers within the PP matrix, and extrusion compounding processing conditions may impact not only the mechanical properties of the matrix polymer, but also the mechanical properties of the PET fibers. Application Ser. No. 11/318,363, filed Dec. 23, 2005, proposes solutions to these problems.
Despite advances in the art, a need exists for a composite interior cover trim panel having improved stiffness, surface finish, impact resistance and flexural modulus characteristics and for a process for making such fiber reinforced polypropylene composite interior cover trim panels.